1. Field of the Invention
The present invention relates to a media access control device guaranteeing Quality of Service (QoS) in a wireless LAN for Voice over IP (VoIP) that can guarantee communication quality of VoIP services provided in wireless LAN areas.
2. Description of the Related Art
Voice over IP (VoIP) is a technology for transferring voice information using an Internet Protocol (IP). This technology provides voice information services in digital form through discrete packets, rather than using a circuit-based protocol as in a Public Switched Telephone Network (PSTN). Attempts have been made to provide the VoIP service not only in wired networks but also in wireless networks. The key to implementation of the VoIP service is to guarantee Quality of Service (QoS) above a certain level with packets that are discretely transferred as described above.
Of various data such as voice, video and Internet data, real-time data including voice data must be processed earlier than other data (i.e., non-real-time data) in order to guarantee QoS in providing the VoIP service in a wireless LAN environment in which a plurality of stations 11 to 13 are connected to a single Access Point (AP) as shown in FIG. 1.
IEEE 802.11 standard uses Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) in the environment as shown in FIG. 1. A station is given an opportunity to access the medium according to Distributed Coordination Function (DCF) timing rules after waiting until the medium becomes idle. A backoff algorithm used in this technique employs a collision avoidance scheme to prevent transmission data from colliding with each other on a channel. A station having data to transmit monitors the channel. If the channel is busy or if it is busy after waiting a time interval corresponding to a DCF Inter-Frame Space (DIFS), the data is transmitted after performing backoff count for a time interval corresponding to a random Contention Window (CW) value generated by a random generator. The random CW value is randomly drawn from the range [0, CWrange] (i.e., the range of 0 to CWrange). The initial random CW value is drawn from the range [0, CWmin]. Each time transmission fails, CWrange is increased to “2(CWrange+1)−1” until it reaches the maximum CW “CWmax” stored in a CWmax register, in which the random CW value is drawn from the increased range.
If each station 11 to 13 fails to transmit a frame, it performs backoff in the above manner to retransmit the frame until the maximum number of times of retransmission is reached. If the next data to transmit is real-time data, transmission delay occurs since the next data cannot be transmitted until the previous data transmission is completed, which makes it difficult to guarantee QoS.
According to the IEEE 802.11e standard, which has been proposed for wireless LAN environments, data is classified into 8 types as shown in FIG. 2. 0th to 2nd priority is assigned to best effort data, 3rd priority is assigned to video probe data, 4th and 5th priority is assigned to video data, and 6th and 7th priority is assigned to voice data. The 8 data types are classified into four Access Categories (AC0 to AC3). As shown in FIG. 3, a Media Access Control (MAC) 30 according to the IEEE 802.11e standard includes four AC processors 31 to 34 for individually processing the four access categories (AC0 to AC3), and four backoff processors 35 to 38 connected respectively with the four AC processors 31 to 34. Each of the four AC processors 31 to 34 includes a single queue to transmit voice, video, or general data according to Enhanced Distributed Channel Access (EDCA) rules. As the priority of data increases, the range of CW values decreases, shortening its backoff time, so that the station more quickly gets the opportunity to transmit the data. Different Arbitration Inter-Frame Space Numbers (AIFSN) are assigned to the access categories, so that the access categories AC3, AC2, AC1 and AC0 are sequentially allocated slot times for starting transmission.
However, in the IEEE 802.11e standard, the processor load is increased to guarantee QoS for each data type, and memory or functional block size is also increased to implement the media access control unit. Although the above method can guarantee QoS for both the real-time and non-real-time data to achieve high quality communication, it is difficult to apply the method to portable devices such as VoIP phones that require small size and low power design, saving hardware resources.